1. Field of the Invention
The present invention relates to profile generating methods and image processing apparatuses, and particularly relates to a method for defining device-independent color spaces used for color conversion using profiles.
2. Description of the Related Art
In recent years, digital apparatuses such as digital still cameras and image scanners have become increasingly popular and digital images are readily obtained. In addition, techniques of full-color hard-copying have been rapidly developed. In particular, a printing quality of an inkjet printing technique rivals that of silver halide photography, and the inkjet printing technique has been widely used. Furthermore, networking systems such as the Internet have become increasingly popular, and a number of users can connect various devices to the network systems. In such a circumstance in which various I/O devices are connected to the network systems, there are several occasions in which a color image displayed on a monitor attaining wide color gamut is hard-copied using a printer having a color reproduction gamut different from that of the monitor, that is, color image data is input and output between devices having different color gamuts.
As a technique of reproducing identical colors using devices employing different color gamuts, a color management system (hereinafter referred to as a “CMS”) is widely known. FIG. 1 is a diagram illustrating an outline of a configuration of the CMS which uses a device-independent color space.
Referring to FIG. 1, when an input device (such as a camera or a scanner) is connected to an output device (such as a printer or a monitor), color signals of an input system are converted into color signals of an output system using corresponding profiles and device-independent color spaces (PCSs). Note that examples of the PCSs include CIEXYZ and CIELab. The profiles are provided as conversion expressions for associating colors employed in the devices with the PCSs or as look-up-tables which are conversion tables including the relationships between the colors employed in the devices and the PCSs described in advance.
In the CMS, when the devices reproduce colors, a gamut compression technique of reducing influence of color gamuts different between the input device and the output device is employed so that the output device reproduces color reproducible using the input device or the input device obtains color reproducible using the output device.
For example, Japanese Patent Laid-Open No. 6-225130 discloses a general gamut compression technique employed in an input device and an output device having different color gamuts. Specifically, Japanese Patent Laid-Open No. 6-225130 discloses a technique of converting an input color space into a uniform color space which is a device-independent color space, and compressing colors in the uniform color space which are not reproducible using the output device in a direction in which a color difference is minimized, and a technique of performing nonlinear compression in accordance with saturation in a direction in which brightness is fixed. Furthermore, Japanese Patent Laid-Open No. 4-40072 discloses a method for converting an input color space into a uniform color space or an HVC (hue, value, and chroma) color space which are device-independent color spaces, and determining whether a color of the converted color space is out of the color gamut of an output-destination device. When it is determined that the color is out of the color gamut, the color is compressed so as to have brightness and hue which are not changed and a saturation of a maximum color value in a reproducible range of the output destination device.
Furthermore, Japanese Patent Laid-Open No. 2004-104603 discloses a technique of compressing a gamut while a hue of a HVC color space is maintained.
However, when the technique of compressing a gamut is performed in a single color space as described above, some hues may not be visually maintained after the compression.
For example, when blue is subjected to gamut compression toward an achromatic axis in a CIELab color space, a phenomenon in which a color obtained after the gamut compression is viewed as purplish blue may occur. Similarly, when blue is subjected to the gamut compression toward the achromatic axis in a CIELuv color space, influence of this phenomenon is reduced. However, when red is subjected to the gamut compression toward the achromatic axis in the CIELuv color space, a phenomenon in which a color obtained after the gamut compression is viewed as a color closer to magenta when compared with a color before gamut compression may occur. When red is subjected to the gamut compression toward the achromatic axis in the CIELab color space, influence of this phenomenon is reduced.
When hues are focally compared with one another as described above, visual constant hue maintaining properties (hereinafter referred to as “visual constant hue properties”) are considerably different among types of color space definitions.
An example of a method for evaluating a visual constant hue property in a color space will be described.
First, four colors, i.e., a primary color of a hue of interest (red in this example), white, black, and gray are selected, and these four colors are subjected to color conversion in a color space (Lab in this example) to be evaluated. Accordingly, color coordinates (an L-value, an a-value, and a b-value in this example) for the four colors are obtained.
Then, the color coordinate of the primary color and the color coordinate of white are compensated for each other so that gradation is generated. Similarly, the color coordinate of the primary color and the color coordinate of black are compensated for each other so that gradation is generated, and the color coordinate of the primary color and the color coordinate of gray are compensated for each other so that gradation is generated. Note that the gradation may have continuous levels or may be a patch having necessary and sufficient steps.
FIG. 3 shows examples of obtained gradation images.
Patches which constitute the corresponding gradation images have an identical hue angle in a color space to be evaluated.
By performing visibility evaluation on the hue angle, a visual constant hue property of the hue of interest is obtained.
Gradation images the same as those obtained in the color space of interest are generated using the four colors in another color space (Luv in this example) and are compared with the gradation images in the color space of interest. In this way, a color space which has a higher visual constant hue property for the hue of interest is determined as evaluation through the comparison between these color spaces.
When the CIELab (hereinafter referred to as an “Lab”), the CIELuv (hereinafter referred to as an “Luv”), and CIECAM02 (JCh) which are widely used color spaces are subjected to main-hue visibility evaluation through the procedure described above, the following results are obtained.
In the vicinity of yellow, the highest visual constant hue property is obtained in the CIELab color space. In the vicinity of blue, the highest visual constant hue property is obtained in the CIELuv color space. In the vicinities of other colors, the highest visual constant hue properties are obtained in the CIECAM02 (JCh).
The visibility evaluation is performed under an environment of a D50 standard light source, and JCh is obtained as an environmental variable specified in the CIECAM02 so as to conform to the environment.
As described above, it is apparent that a visual constant hue property is not sufficiently assured by a hue in color compression using a single color space disclosed in the related art.
Accordingly, there is a demand of a gamut compression method which sufficiently assures the visual constant hue property even after compression of any hue.